Two Objects - One Material Scale
Why can't I just apply the material to any object I want and adjust
the material's scale universally to all objects with the same material
using the &quot;Mapping&quot; Node?

Because the UV maps of the objects can be anything. They can differ in size even on different parts of one object.
Unwrap UVs at the same time
In order to have uniform scale of UVs you can select multiple objects, go into edit mode(2.80 now supports editing multiple objects at once - that's new), select all and unwrap them all at once. If the scale of the objects is applied for all of them, you will get UV islands that will match in scale for all objects as much as it's possible.